A conventional communication system includes a first communication device and a second communication device. The first communication device includes a first control unit, a transmission unit, and an error signal reception unit.
The second communication device includes a second control unit, a reception unit, and a response unit.
The first control unit outputs transmission data to the transmission unit. The transmission unit converts the transmission data output from the first control unit to a first frequency and transmits the converted data to the second communication device (first transmission). The reception unit of the second communication device receives data from the transmission unit. If it is determined that the signal received by the reception unit is not correct transmission data, the second control unit outputs an error signal to the response unit. The response unit converts the error signal from the second control unit to the first frequency and transmits the converted signal to the error signal reception unit of the first communication device. An output of the error signal reception unit is supplied to the first control unit. If the first control unit detects the error signal, the first control unit outputs transmission data again to the transmission unit. Then, the transmission unit performs second transmission. The first control unit outputs the transmission data again after a lapse of a random backoff time after outputting an error signal.
Patent Literature 1, for example, is known as prior art information related to this application.